The present invention relates to a dual mode filter and, more particularly, to such a filter having a plurality of cavities, each of which is capable of supporting two orthogonal and independent resonators.
Dual mode filters of the above-mentioned type are widely known and are used because, as compared to single mode filters, they provide the advantage that they use one-half the number of mechanical cavities, while delivering substantially the same performance as single mode devices. Besides requiring one-half of the number of cavities, dual mode filters provide other advantages such a reduced size and weight and lower cost.
Of the two most common waveguide cross-section shapes are the circular shape used for realizing the TE.sub.111 filter mode and the rectangular cross-section which is used for realizing the TE.sub.101 filter mode.
For clearness, FIG. 1 illustrates by means of a very simplified schematic a single cavity Ci from a series of cavities C1 to Cn. From the TE.sub.111 mode resonator in the form of the circular cavity Ci, it is possible to produce two orthogonal resonators by means of screws V.sub.1 i and V.sub.2 i, spaced 90.degree. from one another with respect to the outer circular periphery of the resonator, by coupling them by means of a third screw V.sub.3 i which is located at a 45.degree. angular position.
The cavities Ci are separated from each other by cross walls PTi, for example as shown in FIG. 2a. To couple adjacent cavities, a small opening Ai is formed in the cross wall PTi. In referring to the openings, such as opening Ai, the terms "coupling opening" or "iris" are used interchangeably. As shown in FIG. 2, the opening Ai is circular and is located in the middle of cross wall PTi. The opening Ai is shown in longitudinal cross-section in FIG. 2a.
In another conventional resonator, the coupling opening Ai is configured as shown in FIGS. 3 and 3a. The opening, sometimes referred to as an iris, comprises four radially directed openings Ai1, Ai2, Ai3, and Ai4 which extend from a center O and which are spaced 90.degree. apart from one another. The radial openings Ai1-Ai4, have respective end boundaries which close the radial end of the opening and which are denoted by reference numerals Vi1, Vi2, Vi3, and Vi4. The ends are located a predetermined distance from the center O. Together, the four radial openings appear as a single cross shaped opening.
The irises shaped and located as described above are encumbered by many disadvantages, several of which are indicated below. A first and significant disadvantage derives from the fact that such an iris does not permit frequency based coupling adjustment. Thus, the filter which is calibrated to different frequencies can never have an equal band width at the various calibration frequencies.
A second disadvantage, growing out the shape of the iris, is that it affects and causes large variations on other filter characteristics and not only in the first frequency related parameter of the filter.
Moreover, such prior art filters also have increased insertion losses, and introduce at the final test level precalibration phases. By this it is meant that each cavity Ci, selected from one of the "n" cavities of the filter must be individually adjusted or precalibrated. And after all of the "n" cavities, identified herein as cavitites Ci to Cn have been assembled, even small disturbances or touches are sufficient to produce a response curve which is quite different from a predicted or desired theoretical curve (i.e. minimum and maximum inband matching is adversely affected).